Path Computation Element Working Group Q. Wu
Internet-Draft Huawei
Intended status: Standards Track July 04, 2013
Expires: January 05, 2014

Path Computation Element (PCE) Discovery using DNS


Discovery of the (Path Computation Element (PCE) within an IGP area or domain is possible using OSPF [RFC5088] and IS-IS [RFC5089]. However, in some deployment scenarios PCEs may not wish, or be able, to participate within the IGP process. Therefore it would be beneficial for the Path Computation Client (PCC) to discover PCEs via an alternative mechanism to those proposed in [RFC5088] and [RFC5089].

This document specifies the requirements, use cases, procedures and extensions to support DNS for PCE discovery.

Status of This Memo

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This Internet-Draft will expire on January 05, 2014.

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Table of Contents

1. Introduction

The Path Computation Element Communication Protocol (PCEP) is a transaction-based protocol carried over TCP. In order to be able to direct path computation requests to the Path Computation Element (PCE), a Path Computation Client (PCC) needs to know the location and capability of a PCE.

In a network where an IGP is used and where the PCE participates in the IGP, discovery mechanisms exist for PCC to learn the identify and capability of each PCE. [RFC5088] defines a PCE Discovery (PCED) TLV carried in an OSPF Router LSA. Similarly, [RFC5089] defines the PCED sub-TLV for use in PCE Discovery using IS-IS.

However in certain scenarios not all PCEs will participate in the IGP instance, section 3 (Motivation) outlines a number of use cases. In these cases, current PCE Discovery mechanisms are therefore not appropriate and another PCE announcement and discovery function would be required.

1.1. Requirements

As described in [RFC4674], the PCE Discovery information should at least be composed of:

that allows PCCs to select appropriate PCEs:

This document specifies an extension to DNS for the above PCE information discovery, which is complimentary to IS-IS extension and OSPF extension that describe the support of PCE discovery.

2. Conventions used in this document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119 [RFC2119].

3. Motivation

This section discusses in more detail the motivation and use cases for an alternative DNS based PCE discovery mechanism.

3.1. Load Sharing of Path Computation Requests

Inherent DNS based load balancing may be used for inbound load balancing and implemented at the application level in both servers and clients. Multiple host IP addresses are configured in DNS for a single host server name. Also DNS is capable of automatically detecting and reacting to errors. These allow you to provide load balancing across two separate Systems and facilitate PCE system failover and recovery.

3.2. Network Address Translation Gateway

Network address translation breaks end-to-end connectivity. The internal network devices communicate with hosts on the external network by changing the source address of outgoing requests to that of the NAT device and relaying replies back to the originating device. This poses a problem for PCEs that is behind the NAT devices, as PCE information carried by PCE that is behind NAT devices in the outgoing IGP advertisement doesn't require substitution or special traversal techniques for NAT traversal and can not be used by PCC that is on the external network to reach the PCE.

3.3. Multiple-Provider Domains

In the case of multiple ASes within different service provider networks, the H-PCE [RFC6805] architecture does not require disclosure of internals of a child domain to the parent PCE. It may be necessary for a third party to manage the parent PCEs according to commercial and policy agreements from each of the participating service providers.

[RFC6805] specifies that a child PCE must be configured with the address of its parent PCE in order for it to interact with its parent PCE. However handling changes in parent PCE identities and coping with failure events would be an issue for a configured system.

There is no scope for parent PCEs to advertise their presence, however there is potential for directory systems (such as DNS [RFC4848] as used in the ALTO discovery function [I-D.ietf-alto- server-discovery]).

4. Discovering a Path Computation Element

The Dynamic Delegation Discovery System (DDDS) [RFC3401] is used to implement lazy binding of strings to data, in order to support dynamically configured delegation systems. The DDDS functions by mapping some unique string to data stored within a DDDS database by iteratively applying string transformation rules until a terminal condition is reached. When DDDS uses DNS as a distributed database of rules, these rules are encoded using the Naming Authority Pointer (NAPTR) Resource Record (RR). One of these rules is the First Well Known Rule, which says where the process starts.

In current specifications, the First Well Known Rule in a DDDS application [RFC3403] is assumed to be fixed, i.e., the domain in the tree where the lookups are to be routed to, is known. This document proposes the input to the First Well Known Rule to be dynamic, based on the search path the resolver discovers or is configured to use.

The search path of the resolver can either be pre-configured, or discovered using DHCP.

When the PCC needs to discover PCEs in the domain into which the PCC has visibility (e.g.,local domain), the input to the First Well Known Rule MUST be the domain the PCC knows, which is assumed to be pre-configured in the PCC or discovered using DHCP.

When the PCC needs to discover PCE in the other domain (e.g., Parent PCE in the parent domain)into which the PCC has no visibility, it SHOULD know the domain name of that domain and use DHCP to discover IP address of the PCE in that domain that provides path computation service along with some PCE location information useful to a PCC for PCE selection, and contact it directly. In some instances, the discovery may result in a per protocol/application list of domain names that are then used as starting points for the subsequent NAPTR lookups. If neither the IP address or PCE location information can be discovered with the above procedure, the PCC MAY request a domain search list, as described in [RFC3397] and [RFC3646], and use it as input to the DDDS application.

When the PCC does not find valid domain names using the procedures above, it MUST stop the attempt to discover any PCE.

The dynamic rule described above SHOULD NOT be used for discovering services other than Path computation services described in this document, unless stated otherwise by a future specification.

The procedures defined here result in an IP address, PCE domain, neighboring PCE domain and PCE Computation Scope where the PCC can contact the PCC that hosts the service the PCC is looking for.

4.1. Determining the PCE Service and transport protocol

The PCC should know the service identifier for the Path Computation Discovery service. The service identifier for the Path Computation Discovery service is defined as "PCED", The PCE supporting "PCED" service MUST support only TCP as transport, as described in [RFC5440].

The services relevant for the task of transport protocol selection are those with NAPTR service fields with values "ID+M2X", where ID is the service identifier defined in the previous section, and X is a letter that corresponds to a transport protocol supported by the domain. This specification only defines M2T for TCP. This document also establishes an IANA registry for mappings of NAPTR service name to transport protocol.

These NAPTR [RFC3403] records provide a mapping from a domain to the SRV [RFC2782] record for contacting a PCE with the specific transport protocol in the NAPTR services field. The resource record MUST contain an empty regular expression and a replacement value, which indicates the domain name where the SRV record for that particular transport protocol can be found. As per [RFC3403], the client discards any records whose services fields are not applicable.

The PCC MUST discard any service fields that identify a resolution service whose value is not "M2T", for values of T that indicate TCP transport protocols supported by the client. The NAPTR processing as described in RFC 3403 will result in the discovery of the most preferred transport protocol of the PCE that is supported by the client, as well as an SRV record for the PCE.

4.2. Determining the IP Address of the PCE

Order Pref Flags  Service     Regexp       Replacement
 IN NAPTR  50   50   "s"  "PCED"    ""
 IN NAPTR  90   50   "s"  "PCED+M2T"    ""

As an example, consider a client that wishes to find “PCED” service in the domain. The client performs a NAPTR query for that domain, and the following NAPTR records are returned:

   ;;  Priority  Weight    Port        Target
IN  SRV    0        1      XXXX
IN  SRV    0        2      XXXX

This indicates that the domain does have a PCE providing Path Computation services over TCP, in that order of preference. Since the client only supports TCP, TCP will be used, targeted to a host determined by an SRV lookup of That lookup would return:

where XXXX represents the port number at which the service is reachable.

Note that the regexp field in the NAPTR example above is empty. The regexp field MUST NOT be used when discovering path computation services, as its usage can be complex and error prone. Also, the discovery of the path computation service does not require the flexibility provided by this field over a static target present in the TARGET field.

If the client is already configured with the information about which transport protocol is used for a path computation service in a particular domain, it can directly perform an SRV query for that specific transport using the service identifier of the path computation Service. For example, if the client knows that it should be using TCP for path computation service, it can perform a SRV query directly

Once the server providing the desired service and the transport protocol has been determined, the next step is to determine the IP address.

According to the specification of SRV RRs in [RFC2782], the TARGET field is a fully qualified domain name (FQDN) that MUST have one or more address records; the FQDN must not be an alias, i.e., there MUST NOT be a CNAME or DNAME RR at this name. Unless the SRV DNS query already has reported a sufficient number of these address records in the Additional Data section of the DNS response (as recommended by [RFC2782]), the PCC needs to perform A and/or AAAA record lookup(s) of the domain name, as appropriate. The result will be a list of IP addresses, each of which can be contacted using the transport protocol determined previously.

4.3. Determining path computation scope,the PCE domains and Neighbor PCE domains

DNS servers MAY add new RRsets to the additional information section that are relevant to the answer and have the same authenticity as the data (the IP Address of the PCE)in the answer section. RRsets include path computation scope, the PCE domains and Neighbor PCE domains associated with. path information Applications on the PCC MAY inspect those Additional Information section and be capable of handling responses from nameservers that never fill in the Additional Information part of a response.

5. IANA Considerations

The usage of NAPTR records described here requires well-known values for the service fields for the transport supported by Path Computation Services. The table of mappings from service field values to transport protocols is to be maintained by IANA.

Service Field: The service field being registered.

Protocol: The specific transport protocol associated with that
service field.  This MUST include the name and acronym for the
protocol, along with reference to a document that describes the
transport protocol.

Name and Contact Information: The name, address, email address,
and telephone number for the person performing the registration.

The registration in the RFC MUST include the following information:

Service Fields                    Protocol
   PCED+M2T                        TCP

The following values have been placed into the registry:

New Service Fields are to be added via Standards Action as defined in [RFC5226].

IANA is also requested to register PCED as service name in the Protocol and Service Names registry.

6. Security Considerations

It is believed that this proposed DNS extension introduces no new security considerations (i.e., A list of known threats to services using DNS) beyond those described in [RFC3833]. For most of those identified threats, the DNS Security Extensions [RFC4033] does provide protection. It is therefore recommended to consider the usage of DNSSEC [RFC4033] and the aspects of DNSSEC Operational Practices [RFC4641] when deploying Path Computation Services.

In deployments where DNSSEC usage is not feasible, measures should be taken to protect against forged DNS responses and cache poisoning as much as possible. Efforts in this direction are documented in [RFC5452].

Where inputs to the procedure described in this document are fed via DHCP, DHCP vulnerabilities can also cause issues. For instance, the inability to authenticate DHCP discovery results may lead to the Path Computation service results also being incorrect, even if the DNS process was secured.

7. References

7.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", March 1997.
[RFC2782] Gulbrandsen, A., "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000.
[RFC3397] Aboba, B., "Dynamic Host Configuration Protocol (DHCP) Domain Search Option", RFC 3397, November 2002.
[RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part Three: The Domain Name System (DNS) Database", RFC 3403, October 2002.
[RFC3646] Droms, R., "DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, December 2003.
[RFC5226] Narten, T., "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 5226, May 2008.
[RFC4674] Droms, R., "Requirements for Path Computation Element (PCE) Discovery", RFC 4674, December 2003.
[RFC6805] King, D. and A. Farrel, "The Application of the Path Computation Element Architecture to the Determination of a Sequence of Domains in MPLS and GMPLS", RFC 6805, November 2012.
[RFC4848] Daigle, D., "Domain-Based Application Service Location Using URIs and the Dynamic Delegation Discovery Service (DDDS)", RFC 4848, April 2007.
[RFC5440] Le Roux, JL., "Path Computation Element (PCE) Communication Protocol (PCEP)", RFC 5440, April 2007.
[RFC4641] Kolkman, O., "DNSSEC Operational Practices", RFC 4641, September 2006.
[RFC4033] Arends, R., "DNS Security Introduction and Requirements", RFC 4033, March 2005.

7.2. Informative References

[RFC5088] Le Roux, JL., "OSPF Protocol Extensions for Path Computation Element (PCE) Discovery", RFC 5088, January 2008.
[RFC5089] Le Roux, JL., "IS-IS Protocol Extensions for Path Computation Element (PCE) Discovery", RFC 5089, January 2008.
[RFC3401] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part One: The Comprehensive DDDS", RFC 3401, October 2002.
[RFC3833] Atkins, D., "Threat Analysis of the Domain Name System (DNS)", RFC 3833, August 2004.
[RFC5452] Hubert, A., "Measures for Making DNS More Resilient against Forged Answers", RFC 5452, January 2009.
[ALTO] Kiesel, S., "ALTO Server Discovery", ID draft-ietf-alto-server-discovery-08, March 2013.

Author's Address

Qin Wu Huawei 101 Software Avenue, Yuhua District Nanjing, Jiangsu 210012 China EMail: